High-Pressure Raman Noncoincidence Effect and Conformation of Alkyl Side Chain in Alkyl Benzoates

Abstract

The Raman noncoincidence effect has been used to investigate the changes in intermolecular interactions induced by applying high pressure to neat liquid alkyl benzoates. The noncoincidence of the carbonyl band of a homologous series of straight chain alkyl benzoates (methyl, ethyl, propyl, butyl, and hexyl benzoate) and a branched chain alkyl benzoate (2-ethylhexyl benzoate) was measured at 20 and 40 °C over the pressure range 1−5000 bar. The density was measured as a function of pressure for all molecules. A transition point dividing the noncoincidence behavior as a function of density into two regions was found, for all molecules except methyl benzoate. Below the transition point, at lower density, the noncoincidence value was relatively insensitive to changes in density, while above the transition point, the noncoincidence value dropped sharply with increasing density. The decrease in the value of noncoincidence above the transition point was interpreted as the change in intermolecular interactions resulting from the conformational change in favor of a “folded” form of the alkyl side chain shielding the carbonyl group. To examine the plausibility of the presence of energetically allowable conformations with a folded alkyl side chain, conformational searches based on molecular mechanics calculations have also been performed. The strain energy of some alkyl benzoate conformers with folded side chain was calculated to be within a few kcal/mol of the global minimum conformation. Both the experimental results and the conformational analysis suggest that the population of the folded conformer increases under high-pressure conditions owing to its compactness.

Effect of Pressure on Local Order in Liquid Dimethyl Sulfoxide

“…Most often it is due to transition dipole-transition dipole coupling. Recently, it has been shown that even pressure-induced conformational changes in molecules can be detected by measuring the NCE as a function of density ,, the NCE in dipolar aprotic liquids can be explained by the presence of molecular clusters in which molecules are oriented in an at least partially ordered way leading to parallel and antiparallel directions of the molecular dipole vectors.…”

Carbon-13 NMR Study of the Effect of Confinement on the Molecular Dynamics of 2-Ethylhexyl Benzoate

“…Our continuing efforts of studying molecular dynamics of complex, viscous liquids were motivated by the need for improved understanding of the functional fluids at the molecular level . In previous studies, the fluid and molecular properties of 2-ethylhexyl benzoate (EHB) have been investigated as a function of temperature and pressure using several experimental techniques. − Comprising both rigid phenyl ring and flexible alkyl chain, EHB has represented a model compound for hydrocarbon-based elastohydrodynamic lubricants . The studies of the molecular dynamics of EHB at various experimental conditions have provided important information on the motion of this model lubricant molecule.…”

Pressure control of enantiodifferentiating photoisomerization of cyclooctenes sensitized by chiral benzenepolycarboxylates. The origin of discontinuous pressure dependence of the optical yield

“…In this connection, Slager et al reported in their high-pressure Raman spectroscopic study that the ester moiety of simple alkyl benzoate folds into a compact structure at the transition pressure, accompanying a discontinuous change in the C᎐ ᎐ O stretching vibration. 8 A similar discontinuous structural change is also likely to occur in the benzenepolycarboxylates employed in this study, and could be the origin of the unusual pressure dependence of product ee. Therefore, we carefully investigated the electronic absorption, fluorescence and circular dichroism spectral behavior of the chiral benzenepolycarboxylate sensitizers at various P, by using a pressure vessel fitted with sapphire or birefringence-free diamond windows.…”